For the equation x + y = 3. How many possible values for x are there if x and y are both whole numbers? A.2
B.3
C.4

Explanation:

The set of whole numbers is {0, 1, 2, 3, ...}

So we include 0 and any positive number that doesn't have decimal or fractional components to it. Negative values are not included.

Whatever you pick for x, the y value must add to it to get 3. Both x and y must be numbers from the set of whole numbers.

There are four cases shown in the bullet points above, so we have four different solutions. We can't move onto x = 4 because the y solution would be y = -1, but -1 is not in the set of whole numbers.